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The Client’s mystery shopping programme consists of visits to each of the client’s ten locations, grouped into five different areas. Each location receives a single visit each month, but on occasion a visit may not take place.
On the completion of each visit, a questionnaire is completed by the mystery shopper that is used to rate the service they received during the visit. The completed questionnaire is assigned a score, which is calculated based on the number of points achieved over the number of points available, normally expressed as a percentage.
Points Achieved x 100 Points Available
= % Score
PHP Development Test
￼￼The Client has requested that an end of year report is generated to summarise the data collected over the previous year. The report should be easy to understand and show as much useful information as possible.
If you execute the following, what does your RDBMS do?
CREATE TABLE t1 (a int); START TRANSACTION; INSERT INTO t1 (a) VALUES (1); START TRANSACTION; INSERT INTO t1 (a) VALUES (2); ROLLBACK; SELECT * FROM t1;
The answer may surprise you.
Clients often ask what the differences are between the various InnoDB isolation levels, or what ACID means. Here are some simple explanations for those that have not yet read the manual and committed it to memory.
Every select operates without locks so you don’t get consistency and might have dirt reads, which are potentially earlier versions of data. So, no ACID support here.
Has consistent reads without locks. Each consistent read, even within the same transaction, sets and reads its own fresh snapshot.
The InnoDB default isolation level for ACID compliance. All reads within the same transaction will be consistent between each other – ie, the C in ACID. All writes will be durable, etc etc.
The No-SQL tag really lumps together a lot of concepts that are in fact as distinct from eachother as they are from SQL/RDBMS.
An object store is not at all similar to Cassandra and Hypertable, which is not at all like an column store. And when looking at BigTable derivatives, it’s quite important to realise that Google actually does joins in middle layers or apps, so while BigTable does not have joins, the apps essentially do use them – I’ve heard it professed that denormalising everything might be a fab idea, but I don’t quite believe in that for all cases, just like I don’t believe in ditching the structured form of RDBMS being the solution.
SQL/RDBMS has had a few decades of dominance now, and has thus become the great “general purpose” tool. With the ascent of all the other tools, it’s definitely worthwhile to look at them, but also realise that[Read more...]
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